The present invention relates generally to the maintenance of steam generators of nuclear reactor power plants and, more particularly to a robotic arm for manipulating service devices relative to the tube sheet of a nuclear steam generator.
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the maintenance of operating steam generators of nuclear reactor power plants. A typical steam generator in a pressurized water nuclear reactor (PWR) includes a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the ends of the tube bundle opposite its U-like curvature, and a dividing plate that cooperates with the tube sheet to form a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle.
The steam generators of the PWR receive both primary and secondary fluids to produce steam for subsequent production of electricity in a conventional manner. The primary fluid, after being heated by circulation through the nuclear reactor core, enters the steam generator through the primary fluid inlet plenum. From its inlet plenum, the primary fluid flows upwardly through the one end of the tube bundle supported by the tube sheet, through its U-like curvature, downwardly through its opposite other end also supported by the tube sheet, and into its outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-shaped tube bundle in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes of the bundle to the secondary fluid surrounding the tube bundle and causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolate from the secondary fluid by the U-shaped walls of the tubes and by the tube sheet, it is important that the tubes and the tube sheet be maintained defect-free so that no leaks will occur in the tubes or in the welds between the tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
It is often necessary to repeatedly inspect the tubes of the bundle or tube sheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the tubes and tube sheet. However, since the primary fluid, which is generally water, contains radioactive corrosion products, the inlet and outlet plena become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operation on the tubes and tube sheet without requiring the entry of working personnel.
As is well known in the art, robotic systems can be used to reduce or eliminate manual operations in certain industrial operations. This reduction in manual operations may often result in significant productivity improvements in the operation. Moreover, in hazardous or limited access environments the use of robotic systems may not only be advantageous but may also be a necessity. For example, in the inspection of nuclear reactor power plants, it is important to be able to limit the time that working personnel are located in a radioactive environment so as to limit the working personnel's radiation exposure. Thus, the use of robotic systems in nuclear power plant maintenance can result in both improved productivity and in decreased radiation exposure.
In robotic arm systems, the elements which power the movements of the robotic arm may be located away from the arm joints (driving the joints by means of chains or belts) or the elements may be located at each joint. The use of actuators located at each joint decreases the compliance of the arm, but reduces the arm's load capability due to the added weight of the actuator on the arm. On the other hand, the use of actuators located remote from the joint reduces the weight of the arm, but increases compliance and decreases the accuracy of the arm's movement. It is therefore desirable that actuators located in the arms be both powerful and light weight. Traditionally, such actuators have been hydraulic-type actuators because no electric actuator could match the torque-to-weight ratios of hydraulic actuators. Hydraulic systems, however, are more difficult to control, are not capable of continuous rotation (vane type), and require a large amount of peripheral equipment (i.e., pumps and accumulators). Also, when used in environments where human access is limited, the possibility of contamination by the hydraulic fluid exists.
Presently, eddy current testing or inspection of the several tubes located within the tube sheet of a nuclear steam generator is carried out many number of times during the expected life of the steam generator. This service is presently carried out by a robotic arm, an example of which is the model SM-10W designed by Westinghouse Electric Corporation. This robotic arm is driven by a custom designed gearbox which over time experiences a great amount of back-lash due to the wear experienced in the gearbox and drive motor. Because accurate and reliable repeatability is required for carrying out the eddy current testing, that is the ability of the robotic arm to accurately and consistently return the testing mechanism to a particular tube location within the steam generator, the robotic arm must experience little wear and fluctuation in its movement over an extended period of time. Applicants have observed that the back-lash experienced in the present robotic arm has proven detrimental to the field performance of these arms. Over time, the present robotic arms lose their ability to accurately and consistently return to a particular tube location and operators have been forced to count the tubes during the eddy current inspection program in order to assure and verify their location
Therefore, there is clearly a pressing need for a light weight high-torque drive mechanism which can be placed at both the primary and secondary joint locations, and which is capable of accurate operation over an extended period of time.